1. Field of the Invention
The present invention relates to a transmission circuit, and more particularly, to a transmission circuit that raises the efficiency of a power amplifier of a portable telephone, etc.
2. Description of the Related Art
A conventional transmission circuit will be described with reference to FIG. 11. A power amplifier 20 has two amplifiers to the one amplifier on the input side of which Vdd1 is supplied as a power supply voltage and to the other amplifier on the output side of which Vdd2 is supplied as a power supply voltage. In order to control the power supply voltage Vdd1 and Vdd2 to maximize the efficiency of the power amplifier 20, a RAM 50, a control unit 30, and a DC/DC converter 40 are provided. A relational expression between the power supply voltage Vdd and an output voltage Pout is stored in the RAM 50, and the control unit 30 controls the DC/DC converter 40 on the basis of the output voltage Pout and the relational expression, and the DC/DC converter 40 lowers the voltage of a battery 60 on the basis of a control signal output from the control unit 30 to output the lowered voltage.
When the information on the output power Pout of the power amplifier 20 is input, the control unit 30 substitutes the value of the output power Pout for the relational expression stored in the RAM 50, so that the input voltage Vdd is calculated for operating the power amplifier 20 to the maximum efficiency. Next, the control unit 30 controls the DC/DC converter 40 so that the output voltage of the DC/DC converter 40 becomes the calculated value. As a result, the DC/DC converter 40 outputs a power supply voltage suitable for operating the power amplifier 20 most efficiently (for example, see Patent Document 1)
[Patent Document 1]
Japanese Unexamined Patent Application Publication No 2002-290247 (FIG. 1)
In the above configuration, since the actual output power is detected and the control unit sets a power supply voltage on the basis of the information on the output power, the power supply voltage that has been supplied until then, is changed, as a result of variations in the output power. Further, since the relationship between the power supply voltage and the information on what the output voltage should be set to is not considered, there is a big problem in that a power supply voltage required for a set arbitrary output power cannot be supplied.